1. Field of the Invention
This invention relates to filter devices, and more particularly, relates to devices especially, though not exclusively, useful for safely entrapping emboli moving in the circulatory system of animals.
2. Brief Description of the Prior Art
Blood clots (emboli) carried in the blood stream often constitute serious threats to health, and in some instances, to life itself, and the reduction of such clots, or their stabilization and arrest against further migration in the circulatory system of the body, are desiderata constantly motivating the development by the medical profession of new techniques for this purpose. Although emboli moving in other portions of the circulatory system can also present grave problems, techniques for preventing emboli from migrating into the pulmonary circulation from the vena cava have received an unusual amount of attention, since the development of this condition is relatively often experienced in post operative patients, and also may occur in phlebitic patients.
An early technique utilized involved ligation of the vena cava to arrest the movement of the embolus, with collateral circulation then being relied upon to provide adequate venal circulation to the heart. From this type of procedure, which involved a major abdominal operation requiring general anaesthesia and extensive surgery within the abdominal cavity, surgeons have progressed to the utilization of harpstring filters, staple plication, smooth, serrated, and channeling external clips, and even more recently, intravascular springs, balloons, and filters. The use of filters implaced in the vascular system provides the obvious advantage over ligation of major blood vessels, such as the vena cava, of not requiring general anaesthesia surgery and laparotomy. Other significant advantages of the filtering technique with respect to ligation will also be well understood by medically trained persons.
A recent proposal in the field of intravascular filters for entrapment and arresting of emboli is that which has been advanced by Mobin-Uddin and associates, and which is described in U.S. Pat. No. 3,540,431. The Mobin-Uddin filter is an umbrella type structure which includes a plurality of expanding struts or ribs carrying points at the divergent ends thereof which impale or engage the walls of the vena cava or other blood vessel when the filter is positioned at the desired location and permitted to expand to its operative state. In the Mobin-Uddin filter, the apex or hub of the device is located upstream from the pointed, divergent ends of the filter, and on occasion it has been found that the points on the struts of the filter do not adequately impale the internal walls of the vena cava. As a result, the filter becomes dislodged with highly dangerous migration and misplacement of the filter into other veins, as well as the occurrence of duodenal and ureteral perforation by the sharp points at the divergent ends of the filter struts. Further, the manner in which the filter is expanded and its orientation in the blood vessel may result in some perforation or penetration by the points at the ends of the filter struts completely through the blood vessel, with consequent undesirable internal hemorrhaging. The web or canopy extending between the struts of the filter also sometimes, when functioning to arrest movement of emboli in the blood vessel, presents a greater occluding effect than is desirable.
A catheter mounted filter device developed by Eichelter and Schenk is reportedly advantageous in that it is removable from the vascular system after it has performed its function. This filter device, however, has large interstices which may allow the escape of a portion of the entrapped emboli, and thus allow pulmonary embolization.
Another device which is designed to provide occlusion of the vena cava is that which is described in Cohn U.S. Pat. No. 3,334,629. The Cohn occlusion device is emplaced in the vena cava in an opposite orientation from the direction of emplacement of the Mobin-Uddin filter. This device offers some advantage in terms of reduced dislodgment by reason of the carriage on the divergent vanes of the device of a series of inclined teeth which impale the walls of the vena cava, and tend to become more firmly seated in the vascular walls under the impress of blood pressure developed by the flow of blood through the vessel. The techniques and devices proposed for emplacement of the Cohn occlusion device are attended by difficulty and some danger. Moreover, the Cohn structure is normally thought of as an occlusive device in the sense that some collateral circulation is often required in order to adequately supply blood to the heart, particularly when the entrapped embolus is large.
Ideally, an intravascular filter for use in the entrapment and stabilization against further migration of emboli should not merely effectively entrap and filter the emboli, but should permit the preservation of adequate blood flow without the requirement for collateral circulation. The latter condition is usually accompanied by the formation of uncontrolled venous collaterals which are potential sources of recurrent embolization. The filter, of whatever type, should show no tendency to propagate thrombus, and should be susceptible to emplacement at a precise predetermined location without undue patient risk, and preferably under local anaesthesia. To the extent that the filter may permit or, in fact contribute to, fibrinolysis of the entrapped thrombi, it will afford a further advantage and beneficial effect over complete or partial occlusion through ligation, and the use of the types of filters or obstructive techniques previously suggested.